Cisco Catalyst 4500 Series, Software Configuration Manual

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30-3
Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
Chapter 30 Configuring 802.1Q Tunneling, VLAN Mapping, and Layer 2 Protocol Tunneling
Configuring 802.1Q Tunneling
When the packet enters the trunk port of the service provider egress switch, the metro tag is again
stripped as the switch processes the packet. However, the metro tag is not added when the packet is sent
out the tunnel port on the edge switch into the customer network. The packet is sent as a normal
802.1Q-tagged frame to preserve the original VLAN numbers in the customer network.
All packets entering the service provider network through a tunnel port on an edge switch are treated as
untagged packets, whether they are untagged or already tagged with 802.1Q headers. The packets are
encapsulated with the metro tag VLAN ID (set to the access VLAN of the tunnel port) when they are
sent through the service provider network on an 802.1Q trunk port. The priority field on the metro tag
is set to the interface class of service (CoS) priority configured on the tunnel port. (The default is zero
if none is configured.)
In Figure 30-1 , Customer A was assigned VLAN 30, and Customer B was assigned VLAN 40. Packets
entering the edge-switch tunnel ports with 802.1Q tags are double-tagged when they enter the service
provider network, with the metro tag containing VLAN ID 30 or 40, appropriately, and the inner tag
containing the original customer VLAN number, for example, VLAN 100. Even if Customers A and B
both have VLAN 100 in their networks, the traffic remains segregated within the service provider
network because the metro tag is different. Each customer controls its own VLAN numbering space,
which is independent of the VLAN numbering space used by other customers and the VLAN numbering
space used by the service provider network.
Configuring 802.1Q Tunneling
These sections describe 802.1Q tunneling configuration:
• 802.1Q Tunneling Configuration Guidelines, page 30-3
•
802.1Q Tunneling and Other Features, page 30-5
• Configuring an 802.1Q Tunneling Port, page 30-5
Note
By default, 802.1Q tunneling is disabled because the default switch port mode is dynamic auto. Tagging
of 802.1Q native VLAN packets on all 802.1Q trunk ports is also disabled.
802.1Q Tunneling Configuration Guidelines
When you configure 802.1Q tunneling, you should always use asymmetrical links for traffic going
through a tunnel and should dedicate one VLAN for each tunnel. You should also be aware of
configuration requirements for native VLANs and maximum transmission units (MTUs). For more
information about MTUs, see the
“System MTU” section on page 30-5 .
Native VLANs
When configuring 802.1Q tunneling on an edge switch, you must use 802.1Q trunk ports for sending
packets into the service provider network. However, packets going through the core of the service
provider network can be carried through 802.1Q trunks, ISL trunks, or nontrunking links. When 802.1Q
trunks are used in these core switches, the native VLANs of the 802.1Q trunks must not match any native
VLAN of the nontrunking (tunneling) port on the same switch because traffic on the native VLAN is not
tagged on the 802.1Q sending trunk port (
Figure 30-3 ).